The present invention relates to keyboard circuit apparatuses, and more particularly to a keyboard circuit apparatus of a membrane-switch type which uses a wiring matrix to perform key scan operation, and which is constituted such that so-called sneak current is detected by means of a resistance matrix.
For the sake of clarifying the background of the invention, the constitution which uses a resistance matrix for the purpose of detecting sneak current will be explained first.
The phenomenon of sneak current is characteristic of key scan operation. Referring to FIG. 1 showing the principle of a keyboard circuit apparatus disclosed in the Japanese Patent Publication 4-47934, a sneak current i flows when keys B, C and D are pressed at the same time, resulting in the closure of a key A being erroneously detected unless no preventive measure is taken.
A keyboard apparatus 10 in FIG. 1 includes a keyboard circuit apparatus 11 and a key closure detection unit 12.
The keyboard circuit apparatus 11 includes X lines (hereinafter, referred to as detection lines) 13, Y lines (hereinafter, referred to as ground scan lines) 14 crossed by the detection lines, key switches 15-1-15-4 and resistive elements 16-1-16-4. Generally, the key switches of the keyboard circuit apparatus 11 are membrane switches.
The key closure detection unit 12 includes: a scan unit 17 for sequentially scanning the ground scan lines 14 so that the ground scan lines 14 are set at the ground potential one at a time; an analog multiplexor 18 for sequentially selecting as input each of the plurality of detection lines 13; and a comparator 19 for comparing the output voltage of the analog multiplexor 18 with a reference voltage Vref.
An intermediate-level voltage Z.sub.V is applied to the detection lines 13. The level of Z.sub.V is such that 0V&lt;Vref&lt;Z.sub.V &lt;5V.
The detection line 13 that is selected by the analog multiplexor 18 is turned into an open circuit, while all the other detection lines 13 are connected to the voltage source providing Z.sub.V.
When the key A is pressed in a normal manner, the key switch 15-1 is closed as shown in FIG. 2A, with the result that the voltage V of the line 20 is substantially zero. Since V&lt;Vref in this case, the comparator outputs a signal.
When the keys B, C and D are pressed at the same time, the key switches 15-2, 15-3 and 15-4 are closed, as shown in FIG. 1 and FIG. 2B.
In this case, the sneak current i flows as indicated in FIG. 1. Thus, the voltage V of the line 20 is not lower than Z.sub.V. It will be noted that V&gt;Vref because Vref&lt;Z.sub.V. The comparator 19 does not output a signal.
In this way, the erroneous detection of the closure of the key A when the keys B, C and D are pressed at the same time is prevented.
In order that the detection of the sneak current be performed properly as described above, the value of resistance of the resistive elements 16-1-16-4 needs to be precisely controlled.
FIGS. 3 and 4 show a keyboard circuit apparatus 30 which is an embodiment disclosed in the Japanese Laid-Open Patent Application 4-47934, FIG. 3 being a perspective view and FIG. 4 being a cross sectional view. FIG. 5 shows a part of the circuit configuration used in the keyboard circuit apparatus 30 of FIG. 3.
The keyboard circuit apparatus 30 is constituted such that a first sheet 31 is laid under a second sheet 32, an insulating spacer 33 intervening between the two sheets.
The first sheet 31 is formed with detection lines 34, fixed contacts 35 and resistive elements 36.
The second sheet 32 is formed with ground scan lines 37 and movable contacts 38.
The resistive elements 36 are formed by printing carbon in zigzag patterns using a screen printing machine.
Since the resistive elements 36 are formed only on the first sheet 31, there is a problem in that, if the first sheet 31 is defectively produced such that the value of resistance of any of the resistive elements 36 is below or above the allowed value due to variations in the printing process, there is no way of redressing the defect, and the defectively produced first sheet 31 has no use but to be discarded as waste.
The cost of the having to discard defective first sheets 31 adds to the cost of the keyboard circuit apparatus 30, resulting in a relatively high cost of the keyboard circuit apparatus 30.
Another reason for poor yield of the first sheet 31 is that the value of resistance of the resistive elements 36 tends to vary, if the resistive elements 36 are formed in zigzag patterns which may encourage the printed resistive material to seep out from the target area.